5



5 The Integumentary System

Skin (Integument)

Consists of three major regions

Epidermis—superficial region

Dermis—middle region

Hypodermis (superficial fascia)—deepest region

Subcutaneous layer deep to skin (not technically part of skin)

Mostly adipose tissue

Epidermis

Keratinized stratified squamous epithelium

Cells of epidermis

Keratinocytes—produce fibrous protein keratin

Melanocytes

10–25% of cells in lower epidermis

Produce pigment melanin

Epidermal dendritic (Langerhans) cells—macrophages that help activate immune system

Tactile (Merkel) cells—touch receptors

Layers of the Epidermis: Stratum Basale (Basal Layer)

Deepest epidermal layer firmly attached to the dermis

Single row of stem cells

Also called stratum germinativum: cells undergo rapid division

Journey from basal layer to surface

Takes 25–45 days

Layers of the Epidermis: Stratum Spinosum (Prickly Layer)

Cells contain a weblike system of intermediate prekeratin filaments attached to desmosomes

Abundant melanin granules and dendritic cells

Layers of the Epidermis: Stratum Granulosum (Granular Layer)

Thin; three to five cell layers in which the cells flatten

Keratohyaline and lamellated granules accumulate

Layers of the Epidermis: Stratum Lucidum (Clear Layer)

In thick skin

Thin, transparent band superficial to the stratum granulosum

A few rows of flat, dead keratinocytes

Layers of the Epidermis: Stratum Corneum (Horny Layer)

20–30 rows of dead, flat, keratinized membranous sacs

Three-quarters of the epidermal thickness

Functions

Protects from abrasion and penetration

Waterproofs

Barrier against biological, chemical, and physical assaults

Dermis

Strong, flexible connective tissue

Cells include fibroblasts, macrophages, and occasionally mast cells and white blood cells

Two layers:

Papillary

Reticular

Layers of the Dermis: Papillary Layer

Papillary layer

Areolar connective tissue with collagen and elastic fibers and blood vessels

Dermal papillae contain:

Capillary loops

Meissner’s corpuscles

Free nerve endings

Layers of the Dermis: Reticular Layer

Reticular layer

~80% of the thickness of dermis

Collagen fibers provide strength and resiliency

Elastic fibers provide stretch-recoil properties

Skin Markings: Friction Ridges

Epidermal ridges lie atop deeper dermal papillary ridges to form friction ridges of fingerprints

Skin Markings: Cleavage Lines

Collagen fibers arranged in bundles form cleavage (tension) lines

Incisions made parallel to cleavage lines heal more readily

Skin Color

Three pigments contribute to skin color:

Melanin

Yellow to reddish-brown to black, responsible for dark skin colors

Produced in melanocytes; migrates to keratinocytes where it forms “pigment shields” for nuclei

Freckles and pigmented moles

Local accumulations of melanin

Skin Color

Carotene

Yellow to orange, most obvious in the palms and soles

Hemoglobin

Responsible for the pinkish hue of skin

Appendages of the Skin

Derivatives of the epidermis

Sweat glands

Oil glands

Hairs and hair follicles

Nails

Sweat Glands

Two main types of sweat (sudoriferous) glands

Eccrine (merocrine) sweat glands—abundant on palms, soles, and forehead

Sweat: 99% water, NaCl, vitamin C, antibodies, dermcidin, metabolic wastes

Ducts connect to pores

Function in thermoregulation

Sweat Glands

Apocrine sweat glands—confined to axillary and anogenital areas

Sebum: sweat + fatty substances and proteins

Ducts connect to hair follicles

Functional from puberty onward (as sexual scent glands?)

Specialized apocrine glands

Ceruminous glands—in external ear canal; secrete cerumen

Mammary glands

Sebaceous (Oil) Glands

Widely distributed

Most develop from hair follicles

Become active at puberty

Sebum

Oily holocrine secretion

Bactericidal

Softens hair and skin

Hair

Functions

Alerting the body to presence of insects on the skin

Guarding the scalp against physical trauma, heat loss, and sunlight

Distribution

Entire surface except palms, soles, lips, nipples, and portions of external genitalia

Hair

Consists of dead keratinized cells

Contains hard keratin; more durable than soft keratin of skin

Hair pigments: melanins (yellow, rust brown, black)

Gray/white hair: decreased melanin production, increased air bubbles in shaft

Hair Follicle

Extends from the epidermal surface into dermis

Two-layered wall: outer connective tissue root sheath, inner epithelial root sheath

Hair bulb: expanded deep end

Hair Follicle

Hair follicle receptor (root hair plexus)

Sensory nerve endings around each hair bulb

Stimulated by bending a hair

Arrector pili

Smooth muscle attached to follicle

Responsible for “goose bumps”

Types of Hair

Vellus—pale, fine body hair of children and adult females

Terminal—coarse, long hair of eyebrows, scalp, axillary, and pubic regions (and face and neck of males)

Types of Hair

Hair Growth

Growth phase (weeks to years) followed by regressive stage and resting phase (1–3 months)

Growth phase varies (6–10 years in scalp, 3–4 months in eyebrows)

Hair Thinning and Baldness

Alopecia—hair thinning in both sexes after age 40

True (frank) baldness

Genetically determined and sex-influenced condition

Male pattern baldness is caused by follicular response to DHT

Structure of a Nail

Scalelike modification of the epidermis on the distal, dorsal surface of fingers and toes

Functions of the Integumentary System

Protection—three types of barriers

Chemical

Low pH secretions (acid mantle) and defensins retard bacterial activity

Functions of the Integumentary System

Physical/mechanical barriers

Keratin and glycolipids block most water and water- soluble substances

Limited penetration of skin by lipid-soluble substances, plant oleoresins (e.g., poison ivy), organic solvents, salts of heavy metals, some drugs

Biological barriers

Dendritic cells, macrophages, and DNA

Functions of the Integumentary System

Body temperature regulation

~500 ml/day of routine insensible perspiration (at normal body temperature)

At elevated temperature, dilation of dermal vessels and increased sweat gland activity (sensible perspirations) cool the body

Cutaneous sensations

Temperature, touch, and pain

Functions of the Integumentary System

Metabolic functions

Synthesis of vitamin D precursor and collagenase

Chemical conversion of carcinogens and some hormones

Blood reservoir—up to 5% of body’s blood volume

Excretion—nitrogenous wastes and salt in sweat

Skin Cancer

Most skin tumors are benign (do not metastasize)

Risk factors

Overexposure to UV radiation

Frequent irritation of the skin

Some skin lotions contain enzymes in liposomes that can fix damaged DNA

Skin Cancer

Three major types:

Basal cell carcinoma

Least malignant, most common

Squamous cell carcinoma

Second most common

Melanoma

Most dangerous

Basal Cell Carcinoma

Stratum basale cells proliferate and slowly invade dermis and hypodermis

Cured by surgical excision in 99% of cases

Squamous Cell Carcinoma

Involves keratinocytes of stratum spinosum

Most common on scalp, ears, lower lip, and hands

Good prognosis if treated by radiation therapy or removed surgically

Melanoma

Involves melanocytes

Highly metastatic and resistant to chemotherapy

Treated by wide surgical excision accompanied by immunotherapy

Melanoma

Characteristics (ABCD rule)

A: Asymmetry; the two sides of the pigmented area do not match

B: Border exhibits indentations

C: Color is black, brown, tan, and sometimes red or blue

D: Diameter is larger than 6 mm (size of a pencil eraser)

Burns

Heat, electricity, radiation, certain chemicals

(

Burn

(tissue damage, denatured protein, cell death)

Immediate threat:

Dehydration and electrolyte imbalance, leading to renal shutdown and circulatory shock

Rule of Nines

Used to estimate the volume of fluid loss from burns

Partial-Thickness Burns

First degree

Epidermal damage only

Localized redness, edema (swelling), and pain

Second degree

Epidermal and upper dermal damage

Blisters appear

Full-Thickness Burns

Third degree

Entire thickness of skin damaged

Gray-white, cherry red, or black

No initial edema or pain (nerve endings destroyed)

Skin grafting usually necessary

Severity of Burns

Critical if:

>25% of the body has second-degree burns

>10% of the body has third-degree burns

Face, hands, or feet bear third-degree burns

Developmental Aspects: Fetal

Ectoderm ( epidermis

Mesoderm ( dermis and hypodermis

Lanugo coat: covering of delicate hairs in 5th and 6th month

Vernix caseosa: sebaceous gland secretion; protects skin of fetus

Developmental Aspects: Adolescent to Adult

Sebaceous gland activity increases

Effects of cumulative environmental assaults show after age 30

Scaling and dermatitis become more common

Developmental Aspects: Old Age

Epidermal replacement slows, skin becomes thin, dry, and itchy

Subcutaneous fat and elasticity decrease, leading to cold intolerance and wrinkles

Increased risk of cancer due to decreased numbers of melanocytes and dendritic cells

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